Light-up Valentine heart on an Arduino Uno

// Light-up Valentine heart: a heart on an 8x8 LED matrix that beats, a big
// beat and a small one, lub-dub, at a pace you set with a knob.
//
//   MAX7219   8x8 matrix module: DIN -> pin 11, CLK -> pin 13, CS -> pin 10,
//             VCC -> 5 V, GND -> GND
//   knob      potentiometer wiper -> A0, ends to 5 V and GND
//
// Each picture is eight bytes, one per row, a 1 for every lit LED, so the big
// heart and the small heart are two short tables. A heartbeat is not one
// thump: it is two close together and then a rest, so the sketch shows the
// big heart, the small one, the big one again, then rests on the small one.
//
// The knob sets the beats a minute, from a calm 50 to an excited 140. Put the
// matrix behind a sheet of red tissue paper in a card or a little frame and
// it glows like one heart instead of 64 dots.
//
// Things to try. Turn the knob. Draw your own picture in BIG or SMALL: a
// smiling face, or your initials.

#include <LedControl.h>

LedControl matrix(11, 13, 10, 1);   // DIN, CLK, CS, one module

const uint8_t BIG[8] = {
  0b01100110,
  0b11111111,
  0b11111111,
  0b11111111,
  0b01111110,
  0b00111100,
  0b00011000,
  0b00000000,
};
const uint8_t SMALL[8] = {
  0b00000000,
  0b00100100,
  0b01111110,
  0b01111110,
  0b00111100,
  0b00011000,
  0b00000000,
  0b00000000,
};

int beats = 0;

void draw(const uint8_t picture[8]) {
  for (int row = 0; row < 8; row++) matrix.setRow(0, row, picture[row]);
}

void setup() {
  Serial.begin(9600);
  matrix.shutdown(0, false);
  matrix.setIntensity(0, 8);
  matrix.clearDisplay(0);
  draw(SMALL);
  Serial.println("Be my Valentine: the heart is beating");
}

void loop() {
  int bpm = map(analogRead(A0), 0, 1023, 50, 140);
  unsigned long beat = 60000UL / bpm;

  // Lub, dub, rest.
  draw(BIG);
  delay(beat / 8);
  draw(SMALL);
  delay(beat / 10);
  draw(BIG);
  delay(beat / 8);
  draw(SMALL);
  beats++;
  if (beats % 10 == 1) {
    Serial.print("Beat ");
    Serial.print(beats);
    Serial.print(" at ");
    Serial.print(bpm);
    Serial.println(" beats a minute");
  }
  delay(beat - beat / 8 - beat / 10 - beat / 8);
}